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About this tool
Inspect, reorder and individually preview the channels in a stereo or surround file.
Multichannel Audio Mapper rewrites the channel layout of an audio or video file using FFmpeg's pan filter, with four mappings: keep the layout unchanged, swap left and right, fold the left channel to mono, or fold the right channel to mono. It runs FFmpeg compiled to WebAssembly inside the page, takes any audio or video input, and writes out a WAV. It exists for the moment you discover an interview was recorded with the useful voice on one channel and hiss on the other.
Open Multichannel Audio Mapper on AltFTool — it loads instantly in your browser.
Pick an audio or video file under 'Source file' — the input accepts audio/* and video/*, so an MP4 or MOV works without demuxing the audio first.
Choose keep, swap-stereo, left-mono or right-mono from the 'Mapping' menu, then press 'Process locally' to run FFmpeg's pan filter in the browser tab.
The remapped audio downloads as altftool-multichannel-audio-mapper.wav, with progress and the FFmpeg log shown in the 'Local processing report' panel.
Mono folds are done with pan=mono|c0=c0 or c0=c1, which selects a source channel outright rather than muting one side and leaving a lopsided stereo file.
You can feed an MP4 or MOV straight in instead of demuxing the audio first, and the remapped result comes back as a WAV.
FFmpeg runs as WebAssembly in the browser tab, so an unreleased recording is processed locally rather than uploaded for conversion.
Keep passes audio through unchanged (an anull filter), swap-stereo exchanges the two channels with pan=stereo|c0=c1|c1=c0, left-mono outputs only the original left channel as mono, and right-mono outputs only the original right channel as mono.
WAV, regardless of what you feed in. That keeps the remap lossless with no second round of lossy encoding, at the cost of a larger file; re-encode to AAC or MP3 afterwards if you need it small.
The four presets here cover stereo and mono operations on the first two channels. A 5.1 file will load, but the mono options take channel 0 or channel 1 only, so a bespoke surround remap needs a custom pan filter rather than these presets.
No. Processing happens in the browser through FFmpeg's WebAssembly build, so the media stays on your device. The FFmpeg core itself, version 0.12.10, is fetched from a public CDN on first use, which means the first run needs a network connection and takes longer than later runs.